A wave gear device is equipped with a rigid internal gear, a flexible external gear disposed inside the rigid internal gear, and a wave generator that flexes the flexible external gear into an elliptical shape, causing it to partially mesh with the rigid internal gear. When the wave generator is rotated by a motor or the like, the position at which both gears mesh moves circumferentially, generating a reduced-speed relative rotation between the two gears that corresponds to the difference in the number of the two gears' teeth. By fixing one gear so that it does not rotate, it is possible to output and transmit reduced-speed rotation from the other gear to the load side.
Depending on the shape of the flexible external gear, wave gear devices can be classified into flat types, cup types and silk-hat types. As shown in FIG. 6, a flat type wave gear device 1 has two annular rigid internal gears 2 and 3 disposed coaxially in parallel, an annular flexible external gear 4 disposed inside the rigid internal gears 2 and 3, and a wave generator 5 for flexing the flexible external gear 4 into an elliptical shape to partially mesh it with the rigid internal gears 2 and 3 and for moving the meshing position circumferentially.
Here, the fixed rigid internal gear 2 has the same number of teeth as the flexible external gear 4, and the other rigid internal gear 3 has 2n (n being a positive integer) more teeth than the flexible external gear 4. Or, the fixed rigid internal gear 2 has 2n more teeth than the flexible external gear 4 and the other rigid internal gear 3 has the same number of teeth as the flexible external gear 4.
Rotating the wave generator 5 generates relative rotation between the rigid internal gear 3 and the flexible external gear 4 that is much slower than the rotation of the wave generator 5, and this reduced rotation is output from the rigid internal gear 3 to the output side.
The design of a flat type wave gear device thus configured is more compact and simple than a cup type or silk-hat type one. Flat, cup and silk-hat type wave gear devices are disclosed by, for example, Patent References 1, 2 and 3, respectively.
[Patent Reference 1] JP 05-172195 A
[Patent Reference 2] JP 08-166052 A
[Patent Reference 3] JP 02-912382 A
Compared to other types such as the cup and silk-hat types, the utility of a flat type wave gear device is limited by its small load capacity.
The flexible external gear transmits the load in engagement with the rigid internal gears as it is elliptically deformed by the elliptical shape of the wave generator inserted therein. Therefore, in order to increase the load capacity it is necessary to increase the tooth root strength. An important element in determining the tooth root strength is the rim thickness of the flexible external gear. However, previously there have been no studies relating to design optimization of the rim thickness of the flexible external gear of a flat type wave gear device.